1. Field of the Invention
This invention relates to a method and apparatus for washing a photomask substrate or a semiconductor wafer employed in the manufacture process of semiconductor devices. More particularly, it relates to a substrate washing method and apparatus whereby it is possible to exfoliate a resist pattern remaining on the substrate.
2. Description of the Related Art
With an increasingly refined design rule for circuit patterns and device arraying patterns on a semiconductor device, contamination by various particles, metal impurities or organic substances generated on or affixed to a photomask substrate or a semiconductor wafer employed in the production process of semiconductor devices has come to affect the product yield or reliability significantly. Thus it becomes necessary to wash off contaminants occasionally affixed to the photomask substrate or the semiconductor wafer during the manufacture process of the semiconductor devices.
As the method for washing the photomask substrate or the semiconductor wafer, dry washing with plasma processing or irradiation with ultra-violet (UV) rays, or a wet washing using a washing solution, have been proposed.
Dry washing is a technique proposed in keeping with the general tendency of the semiconductor process towards a dry process. It has such assets that it is superior in washing uniformity and suffers from re-deposition of contaminants on the substrates to a lesser extent, while it can be integrated to other dry processes, and a drying step may be eliminated. However, it is not suited to removal of contaminants and is unable to remove particles, while it is liable to damage the substrate and produce secondary contamination. Consequently, if dry washing is applied, it becomes necessary to perform subsequent wet washing or rinsing with pure water.
On the other hand, wet washing may be carried out using an inexpensive device, while being superior in throughput and capable of removing plural contaminants simultaneously. Besides, it is possible with the wet washing to perform batch processing and simultaneous washing of the front and reverse sides. Above all, a photomask substrate for light exposure for reduced size projection in which a circuit pattern or device arraying pattern applied is about five times as large as the wafer is usually washed by wet washing.
The simplest washing method consists of immersing the substrate in a washing tank for a pre-set time and mechanically washing the substrate taken out from the tank with a brush. It is however not possible with such method to prevent the foreign matter mixed into the washing tank or affixed to the brush from being deposited on the substrate. Moreover, the longer the substrate is left after it is washed, the greater is the amount of the foreign matter deposited on the substrate.
As a technique for inhibiting the re-deposition to a certain extent, there is known a technique known as spin washing. With this technique, since the washing solution is sprayed onto the substrate held on a rotary block, the washing liquid is perpetually scattered under the centrifugal force so that re-deposition is hardly incurred. Also, rinsing with pure water may be achieved by spraying pure water in the course of the treatment in substitution for the washing liquid, whereas drying may also be achieved by continuing the rotation of the substrate while the supply of pure water is stopped. The washing apparatus described in, for example, the JP Patent Kokai Publication No. 63-15710 (1988) is directed to the spin washing. However, the washing apparatus is bulky in size since it is provided with a mechanism for optical washing by irradiation of UV rays after the termination of the spin washing.
The above-described wet washing may be directly applied to the exfoliation of the resist pattern (by decomposition and removal) if the washing liquid is replaced by the resist exfoliation liquid. However, in the preparation of semiconductor apparatus, there is recently employed a resist material which can hardly be exfoliated with the combination of the conventional resist exfoliation liquid and the washing apparatus, such as chloromethylstyrene (CMS) resist material. For wet exfoliation of the resist material, which is employed for far-UV lithography, electron lithography or X-ray lithography, it is necessary to use a strongly oxidative resist exfoliation liquid, such as a mixed solution of sulfuric acid (H.sub.2 SO.sub.4) and aqueous hydrogen peroxide (H.sub.2 O.sub.2), known as aqueous persulfuric acid. The H.sub.2 SO.sub.4 --H.sub.2 O.sub.2 liquid mixture, abbreviated to SPM in the field of wet washing, is a washing liquid employed for removing organic contaminants. However, even with the above-mentioned liquid mixture, the CMS resist material cannot be exfoliated stably, such that recourse cannot but be had to O.sub.2 plasma washing which is inferior in costs and throughput.